I have chosen to add a post to this topic in order not to start another, very similar to the existing one. Here is my wishlist on (stages of?) possible future development of the project.
1. A full reimplementation of UFO EU, plus removing bugs. For example, the original game does not make use of Alien Reproduction. Containters that look like if they had young aliens inside, do exist in alien bases. The only problem is that the game sees them as "alien alloys" (btw., I am working on a patch to this). I also mean that OpenXCom should use all user-made patches (I am not sure if it uses RMP files now, but if yes, they are buggy and really need to be patched - it should be implemented in the game, without the need of patching UFO EU datafiles).
2. The next step: a full reimplementation of TFTD, as close to the original game as possible but with all needed patches as well (ex. against changing two smallest subs, or against some obvious research tree bugs). On this stage a player would choose which game he wants to play. Perhaps they should be termed "scenarios" rather than different games.
3. (Optional?) transferring some features from one game to another, like "kneel at end of turn/firing" and "drain time units to zero" buttons in the battlescape (also in the UFO EU scenario). Note that some features in UFO seems to be better than in TFTD (e.g. differences on the equipment screen).
4. Implementing (as options) features of XComUtil, UFOextender, TFTDextender etc.
5. Adding (to both games which I call scenarios) a possibility to use independently created resouces (gfx, snd, mus). Then you would not have to get original games to play OpenXCom. All things should look enough similar to the original ones. Perhaps also better graphics (higher resolution), still as an option.
6. Adding new ships, new terrains, perhaps new weapons (as an alternative or even as an option, plus a modification of the research tree if needed). Even now XComUtil offers a possibility to use a list of, say, several types of Large Scout, including the original one and some other ones. Before each mission the game choose by chance which UFO will be Large Scout in the present battle. This way you can have even 100 of different UFO types in the same campaign, divided into 8 main classes. I would see it all only as an option, all the time, together with a possibility to play an (almost) unmodified original scenario.
7. A free battle (even now there is such an option - it goes further than the original game), i.e. free battles, with a user-defined start point. I mean also games with EU aliens under the water, or games with a mixture of UFO and TFTD aliens in the same battle.
8. Using UFO 2000 lua-coded maps (and other features?), at least in free games. Or: added some maps as optional replacement for existing maps in the normal game (the way XComUtil does it).
9. Transferring some advanced ideas between games as an option. I mean, for example, two-phase alien base attacks, melee weapons, or playing with mixed crew in UFO EU (ex. mutons + ethereals in one mission), or real land missions in TFTD (on land maps from EU or created independently).
10. Making a third scenario: you start in EU, develop your soldiers, fight with aliens, but after Cydonia you continue the campaign with a number of oprions: the same soldiers (XComUtil, as I remember, can import soldiers from UFO EU into a new campaign in TFTD). Note that "TFTD after EU" would be just an option. The player would still have a possibility to play EU or TFTD as close to the original as possible.
11. Yet more advanced ideas in TFTD part of the third scenario: ex. using "land" weapons (imported from EU) during land missions, or possibility to shoot off a flying sub over the land, and next to have a normal land mission with TFTD creatures, say, in a forest map. If UFO EU and TFTD scenarios were played one after another, XCom should not forget UFO land weapons in the second part, and aquanauts should still have a possibility to use it against all those monsters from the deep on land missions. Too hard to implement? I hope not, in some years maybe
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12. And the hardest challenge... Making a fourth scenario with both games in one (still as an option!), perhaps with one developed, logic and interesting scenario. Some ideas: you can have 8 land bases and 8 water bases, first you meet only UFOs, and only next also USOs. It is so because Gauss weapons are a result of further development of plasma technology. Appearing of first subs would be the aliens reaction to your success with UFO aliens (in this scenario the brain on Mars sends a tachyon beam much earlier than in the present games). Details could be discussed further. Would it be hard to implement? Perhaps. But I do not think it would be so hard. If you can already play UFO and TFTD scenarios within one game, the game internal procedures should be able to interpret fuctions of both games, given in proper rulesets. Only some additional features will have to be worked out (e.g. a button to switch between land and sea bases in the Geoscape). Many additional needed things, including several links between research trees of both basic scenarios, like plasma weapons being prerequisites of Gauss weapons, should be easy to do in rulesets files, not in the binaries. Yes, it would be a quite different game, just a new quality. Still with an option to play pure EU and pure TFTD separately...
In fact many of the above ideas can be treated as advanced modding. The only problem is that the game engine should allow such a modding...
At the same time I vote against implementing XCom Apocalypse in this project (even if I really like this game too). But it is just another game while UFO and TFTD are basically the same game. Or maybe implement only some features as an option/advanced modding, like rumbling walls, a possibility to crawl, etc.
And if some day somebody please to make an XCom Apocalypse reimplementation for modern computers, it will be just another project...